Liquid-crystalline medium

ABSTRACT

The invention relates to liquid-crystalline mixtures comprising at least one compound of the formula I, 
     
       
         
         
             
             
         
       
     
     in which
 
X, L 1 , L 2 , L 3 , L 4  and alkenyl have the meanings indicated in claim  1 ,
 
in order to increase the pretilt at the polyimide surface,
 
and to
 
liquid-crystalline media comprising one or more compounds of the formula I in electro-optical liquid-crystal displays and in LC lenses.

The present invention relates to a liquid-crystalline medium (LC medium), to the use thereof for electro-optical purposes, and to LC displays containing this medium.

Liquid crystals are used principally as dielectrics in display devices, since the optical properties of such substances can be modified by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SBE (super-birefringence effect) cells and OMI (optical mode interference) cells. The commonest display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure. In addition, there are also cells which work with an electric field parallel to the substrate and liquid-crystal plane, such as, for example, IPS (in-plane switching) cells. TN, STN, FFS (fringe field switching) and IPS cells, in particular, are currently commercially interesting areas of application for the media according to the invention.

The liquid-crystal materials must have good chemical and thermal stability and good stability to electric fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and produce short addressing times, low threshold voltages and high contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematic or cholesteric mesophase for the above-mentioned cells, at the usual operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used as mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy, have to satisfy various requirements depending on the cell type and area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integrated non-linear elements for switching individual pixels (MLC displays), media having large positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high specific resistance, good UV and temperature stability and low vapour pressure are desired.

Matrix liquid-crystal displays of this type are known. Examples of non-linear elements which can be used to individually switch the individual pixels are active elements (i.e. transistors). The term “active matrix” is then used, where a distinction can be made between two types:

-   1. MOS (metal oxide semiconductor) or other diodes on silicon wafers     as substrate. -   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts the display size, since even modular assembly of various part-displays results in problems at the joints.

In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A distinction is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive work is being carried out worldwide on the latter technology.

The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on its inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-capable displays, in which a mosaic of red, green and blue filters is arranged in such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarisers in transmission and are backlit.

The term MLC displays here encompasses any matrix display with integrated non-linear elements, i.e., besides the active matrix, also displays with passive elements, such as varistors or diodes (MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications (for example pocket televisions) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. Besides problems regarding the angle dependence of the contrast and the response times, difficulties also arise in MLC displays due to insufficiently high specific resistance of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, pp. 141 ff., Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, pp. 145 ff., Paris]. With decreasing resistance, the contrast of an MLC display deteriorates, and the problem of after-image elimination may occur. Since the specific resistance of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important in order to obtain acceptable lifetimes. In particular in the case of low-volt mixtures, it was hitherto impossible to achieve very high specific resistance values. It is furthermore important that the specific resistance exhibits the smallest possible increase with increasing temperature and after heating and/or UV exposure. The low-temperature properties of the mixtures from the prior art are also particularly disadvantageous. It is demanded that no crystallisation and/or smectic phases occur, even at low temperatures, and the temperature dependence of the viscosity is as low as possible. The MLC displays from the prior art thus do not satisfy today's requirements.

Besides liquid-crystal displays which use backlighting, i.e. are operated transmissively and if desired transflectively, reflective liquid-crystal displays are also particularly interesting. These reflective liquid-crystal displays use the ambient light for information display. They thus consume significantly less energy than backlit liquid-crystal displays having a corresponding size and resolution. Since the TN effect is characterised by very good contrast, reflective displays of this type can even be read well in bright ambient conditions. This is already known of simple reflective TN displays, as used, for example, in watches and pocket calculators. However, the principle can also be applied to high-quality, higher-resolution active matrix-addressed displays, such as, for example, TFT displays. Here, as already in the trans-missive TFT-TN displays which are generally conventional, the use of liquid crystals of low birefringence (Δn) is necessary in order to achieve low optical retardation (d·Δn). This low optical retardation results in usually acceptably low viewing-angle dependence of the contrast (cf. DE 30 22 818). In reflective displays, the use of liquid crystals of low birefringence is even more important than in transmissive displays since the effective layer thickness through which the light passes is approximately twice as large in reflective displays as in transmissive displays having the same layer thickness.

For TV and video applications, displays having fast response times are required in order to be able to reproduce multimedia content, such as, for example, films and video games, in near-realistic quality. Such short response times can be achieved, in particular, if liquid-crystal media having low values for the viscosity, in particular the rotational viscosity γ₁, and having high optical anisotropy (Δn) are used.

In the case of TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:

-   -   extended nematic phase range (in particular down to low         temperatures)     -   the ability to switch at extremely low temperatures (outdoor         use, automobiles, avionics)     -   increased resistance to UV radiation (longer lifetime)     -   low threshold voltage.

The media available from the prior art do not enable these advantages to be achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired which facilitate greater multiplexability and/or lower threshold voltages and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further widening of the available parameter latitude (clearing point, smectic-nematic transition or melting point, viscosity, dielectric parameters, elastic parameters) is urgently desired.

In particular in the case of LC displays for TV and video applications (for example LCD TVs, monitors, PDAs, notebooks, games consoles), a significant reduction in the response times is desired. A reduction in the layer thickness d (“cell gap”) of the LC medium in the LC cell theoretically results in faster response times, but requires LC media having higher birefringence Δn in order to ensure an adequate optical retardation (d·Δn). However, the LC materials of high birefringence known from the prior art generally also have high rotational viscosity at the same time, which in turn has an adverse effect on the response times.

Highly wavy surface structures in the display or certain polyimide/polyimide process conditions (rubbing length) may, owing to an inadequate pretilt at the polyimide surface, result in display defects, such as, for example, reverse twist or mura (uneven grey-shade distribution). It is necessary here systematically to increase the pretilt of the mixture by choosing a different material, but without significantly impairing other parameters. This often requires a switch of the mixture concept, since the liquid-crystalline substances available often only influence the pretilt slightly.

The object of the present invention is therefore to find liquid-crystalline mixtures which comprise substances which increase the pretilt disproportionately. Furthermore, the mixtures should have fast response times, low rotational viscosities and very high birefringence.

The invention is based on the object of providing media, in particular for MLC, TN, STN, OCB, HT-VA, FFS or IPS displays of this type, which have the desired properties indicated above and do not exhibit the disadvantages mentioned above or only do so to a lesser extent. In particular, the LC media should have fast response times and low rotational viscosities at the same time as high birefringence. In addition, the LC media should have a high clearing point, high dielectric anisotropy and in particular high birefringence.

It has now been found that this object can be achieved if LC media comprising one or more compounds of the formula I are used. The compounds of the formula I result in mixtures having the desired properties indicated above and increase the pretilt at the polyimide surface disproportionately.

The invention thus relates to a liquid-crystalline medium, characterised in that it comprises one or more compounds of the formula I

in which X denotes F, Cl, CN, OCN, SF₅, a fluorinated alkyl radical having 1-6 C atoms, a fluorinated alkoxy radical having 1-6 C atoms, a fluorinated alkenyl radical having 2-6 C atoms, a fluorinated alkylalkoxy radical having 1-6 C atoms, L¹⁻⁴ each, independently of one another, denote H or F, and alkenyl denotes an alkenyl radical having 2-6 C atoms.

Surprisingly, it has been found that LC media comprising compounds of the formula I have a very good ratio of rotational viscosity γ₁ and clearing point, a high value for the optical anisotropy Δ∈ and very high birefringence Δn, as well as fast response times, a low threshold voltage, a high clearing point, high positive dielectric anisotropy and a broad nematic phase range. Furthermore, the compounds of the formula I are very readily soluble in liquid-crystalline media.

The present invention likewise relates to the compounds of the formula I.

The compounds of the formula I have a broad range of applications. Depending on the choice of substituents, they can serve as base materials of which liquid-crystalline media are predominantly composed; however, liquid-crystalline base materials from other classes of compound can also be added to the compounds of the formula I in order, for example, to modify the dielectric and/or optical anisotropy of a dielectric of this type and/or to optimise its threshold voltage and/or its viscosity.

Preferred compounds of the formula I are, in particular, those in which X denotes fluorine, furthermore OCF₃.

In the compounds of the formula I, “alkenyl” preferably denotes a straight-chain alkenyl radical having 2-5 C atoms, in particular CH₃CH═CHC₂H₄, furthermore H₂C═CHC₂H₄, CH₃CH═CH, H₂C═CH.

L¹⁻⁴ preferably have the meanings L¹=L²=L³=L⁴=F, furthermore L¹=L²=L³=F and L⁴=H, or L¹=L³=F and L²=L⁴=H.

Preferred compounds of the formula I are indicated below:

Of the said compounds, particular preference is given to the compounds of the formulae I2 and I5, very particularly those where X═F.

Particularly preferred compounds are indicated below:

Particular preference is given to the compounds of the formulae I2-1 and I5-1.

In the pure state, the compounds of the formula I are colourless and form liquid-crystalline mesophases in a temperature range which is favourably located for electro-optical use. They are stable chemically, thermally and to light.

The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for the said reactions. Use can also be made here of variants known per se which are not mentioned here in greater detail.

Preferably the compounds of the formula I are prepared as follows:

Further preferred embodiments of the mixtures according to the invention are indicated below:

-   -   The medium additionally comprises one or more neutral compounds         of the formulae II and/or III:

-   -   in which     -   A denotes 1,4-phenylene or trans-1,4-cyclohexylene,     -   a denotes 0 or 1,     -   R³ denotes alkenyl having 2 to 9 C atoms, and     -   R⁴ denotes a straight-chain alkyl radical having 1 to 12 C atoms         or alkenyl radical having 2 to 9 C atoms,     -   The compounds of the formula II are preferably selected from the         following formulae:

-   -   in which R^(3a) and R^(4a) each, independently of one another,         denote H, CH₃, C₂H₅ or C₃H₇, and “alkyl” denotes a         straight-chain alkyl group having 1 to 8 C atoms. Particular         preference is given to compounds of the formulae IIa and IIf, in         particular in which R^(3a) denotes H or CH₃, and compounds of         the formula IIc, in particular in which R^(3a) and R^(4a) denote         H, CH₃ or C₂H₅.     -   Preference is furthermore given to compounds of the formula II         which have a non-terminal double bond in the alkenyl side chain:

-   -   Very particularly preferred compounds of the formula II are the         compounds of the formulae

-   -   Besides one or more compounds of the formula I, the         liquid-crystalline media according to the invention particularly         preferably comprise 5-70% by weight of compounds of the formula

-   -   The compounds of the formula III are preferably selected from         the following formulae:

-   -   in which “alkyl” and R^(3a) have the meanings indicated above,         and R^(3a) preferably denotes H or CH₃. Particular preference is         given to compounds of the formula IIIb;     -   The medium preferably additionally comprises one or more         compounds selected from the following formulae IV to VIII:

-   -   in which     -   R⁰ has the meanings indicated in formula I, and     -   X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy         radical, in each case having 1 to 6 C atoms, a mono- or         polyfluorinated alkenyl or alkenyloxy radical, in each case         having 2 to 6 C atoms.     -   Y¹⁻⁶ each, independently of one another, denote H or F,     -   Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—,         —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO—, —CF₂O— or —OCF₂—, in the         formulae V and VI also a single bond, and     -   r denotes 0 or 1.     -   In the above formulae, X⁰ is preferably F, Cl or a mono- or         polyfluorinated alkyl or alkoxy radical having 1, 2 or 3 C atoms         or a mono- or polyfluorinated alkenyl radical or alkenyloxy         radical having 2 or 3 C atoms. X⁰ is particularly preferably F,         Cl, CF₃, CHF₂, OCF₃, OCHF₂, OCFHCF₃, OCFHCHF₂, OCFHCHF₂,         OCF₂CH₃, OCF₂CHF₂, OCF₂CHF₂, OCF₂CF₂CHF₂, OCF₂CF₂CH₂F,         OCFHCF₂CF₃, OCFHCF₂CHF₂, OCH═CF₂, OCF═CF₂, OCF₂CHFCF₃,         OCF₂CF₂CF₃, OCF₂CF₂CClF₂, OCClFCF₂CF₃, CF═CF₂, CF═CHF, OCH═CF₂,         OCF═CF₂, or CH═CF₂. X⁰ very particularly preferably denotes F or         OCF₃.     -   In the compounds of the formulae IV to VIII, X⁰ preferably         denotes F or OCF₃, furthermore OCHF₂, CF₃, CF₂H, Cl, OCH═CF₂. R⁰         is preferably straight-chain alkyl or alkenyl having up to 6 C         atoms.     -   The compounds of the formula IV are preferably selected from the         following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.     -   Preferably, R⁰ in formula IV denotes alkyl having 1 to 8 C atoms         and X⁰ denotes F, Cl, OCHF₂ or OCF₃, furthermore OCH═CF₂. In the         compound of the formula IVb, R⁰ preferably denotes alkyl or         alkenyl. In the compound of the formula IVd, X⁰ preferably         denotes Cl, furthermore F.     -   The compounds of the formula V are preferably selected from the         following formulae Va to Vj:

-   -   in which R⁰ and X⁰ have the meanings indicated above.         Preferably, R⁰ in formula V denotes alkyl having 1 to 8 C atoms         and X⁰ denotes F;     -   The medium comprises one or more compounds of the formula VI-1

-   -   particularly preferably those selected from the following         formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.         Preferably, R⁰ in formula VI denotes alkyl having 1 to 8 C atoms         and X⁰ denotes F, furthermore OCF₃ and CF₃.     -   The medium comprises one or more compounds of the formula VI-2

-   -   particularly preferably those selected from the following         formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.     -   Preferably, R⁰ in formula VI denotes alkyl having 1 to 8 C atoms         and X⁰ denotes F;     -   The medium preferably comprises one or more compounds of the         formula VII in which Z⁰ denotes —CF₂O—, —CH₂CH₂— or —COO—,         particularly preferably those selected from the following         formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.         Preferably, R⁰ in formula VII denotes alkyl having 1 to 8 C         atoms and X⁰ denotes F, furthermore OCF₃.     -   The compounds of the formula VIII are preferably selected from         the following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above. R⁰ in         formula VIII preferably denotes a straight-chain alkyl radical         having 1 to 8 C atoms. X⁰ preferably denotes F.     -   The medium additionally comprises one or more compounds of the         following formula:

-   -   in which R⁰, X°, Y¹ and Y² have the meaning indicated above, and

each, independently of one another, denote

-   -   where rings A and B do not both simultaneously denote         cyclohexylene;     -   The compounds of the formula IX are preferably selected from the         following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.         Preferably, R⁰ in formula IX denotes alkyl having 1 to 8 C atoms         and X⁰ denotes F. Particular preference is given to compounds of         the formula IXa;     -   The medium additionally comprises one or more compounds selected         from the following formulae:

-   -   in which R⁰, X⁰ and Y¹⁻⁴ have the meaning indicated in formula         I, and

each, independently of one another, denote

-   -   The compounds of the formulae X and XI are preferably selected         from the following formulae:

-   -   in which R⁰ and X⁰ have the meanings indicated above.         Preferably, R⁰ denotes alkyl having 1 to 8 C atoms and X⁰         denotes F. Particularly preferred compounds are those in which         Y¹ denotes F and Y² denotes H or F, preferably F.     -   The medium additionally comprises one or more compounds of the         following formula:

-   -   in which R¹ and R² each, independently of one another, denote         n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl,         each having up to 9 C atoms, and preferably each, independently         of one another, denote alkyl having 1 to 8 C atoms. Y¹ denotes H         or F.     -   Preferred compounds of the formula XII are the compounds of the         formulae

-   -   in which     -   alkyl and alkyl* each, independently of one another, denote a         straight-chain alkyl radical having 1 to 6 C atoms, and     -   alkenyl and     -   alkenyl* each, independently of one another, denote a         straight-chain alkenyl radical having 2 to 6 C atoms.     -   Particular preference is given to the compounds of the formulae         XII-1 and XII-3.     -   A particularly preferred compound of the formula XII-3 is the         compound of the formula XII-3a:

-   -   The compounds of the formula XII are preferably employed in         amounts of 3-30% by weight.     -   The medium additionally comprises one or more compounds selected         from the following formulae:

-   -   in which R⁰, X⁰, Y¹ and Y² have the meanings indicated above.         Preferably, R⁰ denotes alkyl having 1 to 8 C atoms and X⁰         denotes F or Cl;     -   The compounds of the formulae XIII and XV are preferably         selected from compounds of the formulae

-   -   in which R⁰ and X⁰ have the meanings indicated above. R⁰         preferably denotes alkyl having 1 to 8 C atoms. In the compounds         of the formula XIII, X⁰ preferably denotes F or Cl.     -   The medium additionally comprises one or more compounds of the         formulae D1, D2 and/or D3:

-   -   in which Y¹, Y², R⁰ and X⁰ have the meaning indicated above.         Preferably, R⁰ denotes alkyl having 1 to 8 C atoms and X⁰         denotes F.     -   Particular preference is given to compounds of the formulae

-   -   in which R⁰ has the meanings indicated above and preferably         denotes straight-chain alkyl having 1 to 6 C atoms, in         particular C₂H₅, n-C₃H₇ or n-C₅H₁₁.     -   The medium additionally comprises one or more compounds of the         following formula:

-   -   in which Y¹, R¹ and R² have the meanings indicated above. R¹ and         R² preferably each, independently of one another, denote alkyl         having 1 to 8 C atoms.     -   The medium additionally comprises one or more compounds of the         following formula:

-   -   in which X⁰, Y¹ and Y² have the meanings indicated above, and         “alkenyl” denotes C₂₋₇-alkenyl. Particular preference is given         to compounds of the following formula:

-   -   in which R^(3a) has the meaning indicated above and preferably         denotes H;     -   The medium additionally comprises one or more tetracyclic         compounds selected from the formulae XIX to XXIX:

-   -   in which Y¹⁻⁴, R⁰ and X⁰ each, independently of one another,         have one of the meanings indicated above. X⁰ is preferably F,         Cl, CF₃, OCF₃ or OCHF₂. R⁰ preferably denotes alkyl, alkoxy,         oxaalkyl, fluoroalkyl or alkenyl, each having up to 8 C atoms.     -   In the formulae indicated above and below,

preferably denotes

-   -   R⁰ and R⁰* in the formulae indicated above and below are         preferably straight-chain alkyl or alkenyl having 2 to 7 C         atoms;     -   X⁰ in the formulae indicated above and below is preferably F,         furthermore OCF₃, Cl or CF₃;     -   The medium preferably comprises one, two or three compounds of         the formula I, in particular at least one compound of the         formula I2 and/or I5;     -   The medium preferably comprises one or more compounds selected         from the group of the compounds of the formulae I, II, III, V,         VI-1, VI-2, XII, XIII, XIV, XVII, XXIII, XXV;     -   The medium preferably comprises one or more compounds of the         formula VI-1;     -   The medium preferably comprises one or more compounds of the         formula VI-2;     -   The medium preferably comprises 0.5-10% by weight, preferably         1-5% by weight, particularly preferably 1-3% by weight, of         compounds of the formula I;     -   The proportion of compounds of the formulae II-XXVII in the         mixture as a whole is preferably 20 to 99.5% by weight;     -   The medium preferably comprises 25-80% by weight, particularly         preferably 30-70% by weight, of compounds of the formulae II         and/or III;     -   The medium preferably comprises 5-40% by weight, particularly         preferably 10-30% by weight, of compounds of the formula V;     -   The medium preferably comprises 3-30% by weight, particularly         preferably 6-25% by weight, of compounds of the formula VI-1;     -   The medium preferably comprises 2-30% by weight, particularly         preferably 4-25% by weight, of compounds of the formula VI-2;     -   The medium comprises 5-40% by weight, particularly preferably         10-30% by weight, of compounds of the formula XII;     -   The medium preferably comprises 1-25% by weight, particularly         preferably 2-15% by weight, of compounds of the formula XIII;     -   The medium preferably comprises 5-45% by weight, particularly         preferably 10-35% by weight, of compounds of the formula XIV;     -   The medium preferably comprises 1-20% by weight, particularly         preferably 2-15% by weight, of compounds of the formula XVI;     -   The medium additionally comprises one or more compounds of the         formulae St-1 to St-3:

-   -   in which R⁰, Y¹, Y² and X⁰ have the meanings indicated above. R⁰         preferably denotes straight-chain alkyl, preferably having 1-6 C         atoms. X⁰ is preferably F or OCF₃. Y¹ preferably denotes F. Y²         preferably denotes F. Preference is furthermore given to         compounds in which Y¹=F and Y²=H. The compounds of the formulae         St-1 to St-3 are preferably employed in the mixtures according         to the invention in concentrations of 3-30% by weight, in         particular 5-25% by weight.     -   The medium additionally comprises one or more pyrimidine or         pyridine compounds of the formulae Py-1 to Py-5:

-   -   in which R⁰ preferably denotes straight-chain alkyl having 2-5 C         atoms. x denotes 0 or 1, preferably x=1. Preferred mixtures         comprise 3-30% by weight, in particular 5-20% by weight, of         these pyri(mi)dine compound(s).

It has been found that even a relatively small proportion of compounds of the formula I mixed with conventional liquid-crystal materials, but in particular with one or more compounds of the formulae II to XXVII, results in a significant increase in the light stability and in low birefringence values, with broad nematic phases with low smectic-nematic transition temperatures being observed at the same time, improving the shelf life. At the same time, the mixtures exhibit very low threshold voltages and very good values for the VHR on exposure to UV.

The term “alkyl” or “alkyl*” in this application encompasses straight-chain and branched alkyl groups having 1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having 1-6 carbon atoms are generally preferred.

The term “alkenyl” or “alkenyl*” in this application encompasses straight-chain and branched alkenyl groups having 2-7 carbon atoms, in particular the straight-chain groups. Preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples of particularly preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.

The term “fluoroalkyl” in this application encompasses straight-chain groups having at least one fluorine atom, preferably a terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl. However, other positions of the fluorine are not excluded.

The term “oxaalkyl” or “alkoxy” in this application encompasses straight-chain radicals of the formula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m each, independently of one another, denote 1 to 6. m may also denote 0. Preferably, n=1 and m=1-6 or m=0 and n=1-3.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressing times, the threshold voltage, the steepness of the transmission characteristic lines, etc., can be modified in the desired manner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and the like generally result in shorter addressing times, improved nematic tendencies and a higher ratio between the elastic constants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxy radicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generally give lower threshold voltages and lower values of k₃₃/k₁₁ compared with alkyl and alkoxy radicals. The mixtures according to the invention are distinguished, in particular, by high K₁ values and thus have significantly faster response times than the mixtures from the prior art.

The optimum mixing ratio of the compounds of the above-mentioned formulae depends substantially on the desired properties, on the choice of the components of the above-mentioned formulae and on the choice of any further components that may be present.

Suitable mixing ratios within the range indicated above can easily be determined from case to case.

The total amount of compounds of the above-mentioned formulae in the mixtures according to the invention is not crucial. The mixtures can therefore comprise one or more further components for the purposes of optimisation of various properties. However, the observed effect on the desired improvement in the properties of the mixture is generally greater, the higher the total concentration of compounds of the above-mentioned formulae.

In a particularly preferred embodiment, the media according to the invention comprise compounds of the formulae IV to VIII in which X⁰ denotes F, OF₃, OCF₃, OCHF₂, OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic action with the compounds of the formula I results in particularly advantageous properties. In particular, mixtures comprising compounds of the formulae I and VI, or I and XI, or I and VI and XI are distinguished by their low threshold voltages.

The individual compounds of the above-mentioned formulae and the sub-formulae thereof which can be used in the media according to the invention are either known or can be prepared analogously to the known compounds.

The invention also relates to electro-optical displays, such as, for example, TN, STN, TFT, OCB, IPS, FFS, HT-VA or MLC displays, having two plane-parallel outer plates, which, together with a frame, form a cell, integrated non-linear elements for switching individual pixels on the outer plates, and a nematic liquid-crystal mixture having positive dielectric anisotropy and high specific resistance located in the cell, which contain media of this type, and to the use of these media for electro-optical purposes.

Furthermore, the mixtures according to the invention are also suitable for positive VA applications, also referred to as HT-VA applications. These are taken to mean electro-optical displays having an in-plane drive electrode configuration and homeotropic arrangement of the liquid-crystal medium having positive anisotropy.

The mixtures according to the invention are particularly suitable for TN-TFT display applications having a low operation voltage, i.e. particularly preferably for notebook applications.

The liquid-crystal mixtures according to the invention enable a significant broadening of the available parameter latitude. The achievable combinations of clearing point, viscosity at low temperature, thermal and UV stability and high optical anisotropy are far superior to previous materials from the prior art.

The mixtures according to the invention are particularly suitable for mobile applications and high-Δn TFT applications, such as, for example, PDAs, notebooks, LCD TVs and monitors, and also owing to their high birefringence for 3D lens applications.

The liquid-crystal mixtures according to the invention, while retaining the nematic phase down to −20° C. and preferably down to −30° C., particularly preferably down to −40° C., and the clearing point ≧70° C., preferably ≧75° C., at the same time allow rotational viscosities γ₁ of ≦120 mPa·s, particularly preferably 100 mPa·s, to be achieved, enabling excellent MLC displays having fast response times to be achieved.

The dielectric anisotropy Δ∈ of the liquid-crystal mixtures according to the invention is preferably ≧+8, particularly preferably ≧+12. In addition, the mixtures are characterised by low operating voltages. The threshold voltage of the liquid-crystal mixtures according to the invention is preferably ≦1.5 V, in particular ≦1.2 V.

The birefringence Δn of the liquid-crystal mixtures according to the invention is preferably ≧0.08, in particular ≧0.10 and particularly preferably ≧0.11.

The nematic phase range of the liquid-crystal mixtures according to the invention preferably has a width of at least 90°, in particular at least 100°. This range preferably extends at least from −25° C. to +70° C.

If the mixtures according to the invention are used in FFS applications, the mixtures preferably have a value of the dielectric anisotropy of 3-12 and a value of the optical anisotropy of 0.07-0.13.

It goes without saying that, through a suitable choice of the components of the mixtures according to the invention, it is also possible for higher clearing points (for example above 100° C.) to be achieved at higher threshold voltages or lower clearing points to be achieved at lower threshold voltages with retention of the other advantageous properties. At viscosities correspondingly increased only slightly, it is likewise possible to obtain mixtures having a higher Δ∈ and thus low thresholds. The MLC displays according to the invention preferably operate at the first Gooch and Tarry transmission minimum [C. H. Gooch and H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975], where, besides particularly favourable electro-optical properties, such as, for example, high steepness of the characteristic line and low angle dependence of the contrast (German patent 30 22 818), lower dielectric anisotropy is sufficient at the same threshold voltage as in an analogous display at the second minimum. This enables significantly higher specific resistance values to be achieved using the mixtures according to the invention at the first minimum than in the case of mixtures comprising cyano compounds. Through a suitable choice of the individual components and their proportions by weight, the person skilled in the art is able to set the birefringence necessary for a pre-specified layer thickness of the MLC display using simple routine methods.

The structure of the MLC display according to the invention from polarisers, electrode base plates and surface-treated electrodes corresponds to the usual design for displays of this type. The term usual design is broadly drawn here and also encompasses all derivatives and modifications of the MLC display, in particular including matrix display elements based on poly-Si TFTs or MIM.

A significant difference between the displays according to the invention and the hitherto conventional displays based on the twisted nematic cell consists, however, in the choice of the liquid-crystal parameters of the liquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner conventional per se, for example by mixing one or more compounds of the formula I with one or more compounds of the formulae II-XXVII or with further liquid-crystalline compounds and/or additives. In general, the desired amount of the components used in the smaller amount is dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing.

The dielectrics may also comprise further additives known to the person skilled in the art and described in the literature, such as, for example, UV stabilisers, such as Tinuvin® from Ciba Chemicals, antioxidants, free-radical scavengers, nanoparticles, etc. For example, 0-15% of pleochroic dyes or chiral dopants can be added. Suitable stabilisers and dopants are mentioned below in Tables C and D.

In order to set the desired tilt angle locally, polymerisable compounds may also additionally be added to the mixtures according to the invention. Preferred polymerisable compounds are listed in Table E.

In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the trans-formation into chemical formulae taking place in accordance with Table A. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) are straight-chain alkyl radicals having n and m C atoms respectively; n, m and k are integers and preferably denote 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is indicated. In individual cases, the acronym for the parent structure is followed, separated by a dash, by a code for the substituents R¹*, R²*, L¹* and L²*:

Code for R¹*, R²*, L¹*, L²*, L³* R¹* R²* L¹* L²* nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN.F C_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F H H nF.F C_(n)H_(2n+1) F F H nF.F.F C_(n)H_(2n+1) F F F nOCF₃ C_(n)H_(2n+1) OCF₃ H H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H nV-Vm C_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1) H H

Preferred mixture components are shown in Tables A and B.

TABLE A

TABLE B

Particular preference is given to liquid-crystalline mixtures which, besides the compounds of the formula I, comprise at least one, two, three, four or more compounds from Table B.

TABLE C Table C indicates possible dopants which are generally added to the mixtures according to the invention. The mixtures preferably comprise 0-10% by weight, in particular 0.01-5% by weight and particularly preferably 0.01-3% by weight of dopants.

TABLE D Stabilisers which can be added, for example, to the mixtures according to the invention in amounts of 0-10% by weight are mentioned below.

TABLE E Polymerisable compounds which can be added, for example, to the mixtures according to the invention in amounts of 0.5-5% by weight are given below. If necessary, an initiator must also be added for the polymerisation in amounts of 0-1%.

RM-1

RM-2

RM-3

RM-4

RM-5

RM-6

RM-7

RM-8

RM-9

RM-10

RM-11

RM-12

RM-13

RM-14

RM-15

RM-16

RM-17

RM-18

RM-19

RM-20

RM-21

RM-22

RM-23

RM-24

RM-25

RM-26

RM-27

RM-28

RM-29

RM-30

RM-31

RM-32

RM-33

The following examples are intended to explain the invention without restricting it.

EXAMPLES

“Conventional work-up” means: water is added if necessary, the mixture is extracted with methylene chloride, diethyl ether, methyl tert-butyl ether or toluene, the phases are separated, the organic phase is dried and evaporated, and the product is purified by distillation under reduced pressure or crystallisation and/or chromatography. The following abbreviations are used:

DAST diethylaminosulfur trifluoride DCC dicyclohexylcarbodiimide DDQ dichlorodicyanobenzoquinone DIBALH diisobutylaluminium hydride KOT potassium tertiary-butoxide RT room temperature THF tetrahydrofuran pTSOH p-toluenesulfonic acid TMEDA tetramethylethylenediamine

Example 1

Step 1.1

0.13 mol of isopropylmagnesium chloride (2 mol in THF) is added to 0.01 mol of magnesium in 700 ml of dry THF at 20° C. The mixture is stirred for a further 20 min, and a solution of 0.11 mol of A in 200 ml of THF is then added dropwise at 20° C.

After 1 h at room temperature, a solution of 0.13 mol of formylpiperidine in 100 ml of THF is then added dropwise at the same temperature. The mixture is stirred at room temperature for a further 1 h and then hydrolysed using 300 ml of 1 N HCl. The product is isolated by extraction in a conventional manner and recrystallised from n-heptane at −20° C.

Step 1.2

0.053 mol of ethyltriphenylphosphonium bromide is suspended in 80 ml of THF, and 0.05 mol of potassium tertiary-butoxide, dissolved in 50 ml of THF, is added dropwise at 0° C. After 30 min at 0° C., 0.045 mol of B, dissolved in THF, is added dropwise at this temperature. After hydrolysis using water, the mixture is worked up by extraction in a conventional manner, and the product (Z/E mixture) is isolated by chromatography on silica gel using hexane.

This Z/E mixture is dissolved in 40 ml of butyronitrile, 14 ml of 1 M HCl are added, and 3 portions of benzenesulfinic acid sodium salt (0.005 mol each) are added at intervals of 4 h at 60° C. The mixture is stirred for a further 8 h and worked up by extraction in a conventional manner, and the product is isolated by chromatography on silica gel using hexane. The pure E product is obtained by recrystallisation from ethanol.

The following compounds of the formula

are prepared analogously:

L¹ L² L³ L⁴ X H H H H F F H H H F F F H H F F F F H F F H F H F F H F F F H H F F F H H H H OCF₃ F H H H OCF₃ F F H H OCF₃ F F F H OCF₃ F F F F OCF₃ F H F H OCF₃ F H F F OCF₃ H H F F OCF₃ H H H H Cl F H H H Cl F F H H Cl F F F H Cl F F F F Cl F H F H Cl F H F F Cl H H F F Cl H H H H CN F H H H CN F F H H CN F F F H CN F F F F CN F H F H CN F H F F CN H H F F CN H H H H OCHF₂ F H H H OCHF₂ F F H H OCHF₂ F F F H OCHF₂ F F F F OCHF₂ F H F H OCHF₂ F H F F OCHF₂ H H F F OCHF₂ H H H H OCH═CF₂ F H H H OCH═CF₂ F F H H OCH═CF₂ F F F H OCH═CF₂ F F F F OCH═CF₂ F H F H OCH═CF₂ F H F F OCH═CF₂ H H F F OCH═CF₂ H H H H OCF═CF₂ F H H H CF₃ F F H H CF₃ F F F H CF₃ F F F F CF₃ F H F H CF₃ F H F F CF₃ H H F F CF₃ H H H H CF₃

Example 2

0.12 mol of magnesium turnings is initially introduced in 10 ml of THF, and a solution of 0.12 mol of E-5-bromopent-2-ene in THF is added at the boiling point. The mixture is heated at the boil for a further 1 h, and 0.13 mol of anhydrous zinc chloride, dissolved in THF, is then added. 0.08 mol of A, dissolved in THF, is subsequently added dropwise without cooling. After addition of 0.0024 mol of Pd-DPPF catalyst, the temperature rises to about 57° C. The mixture is heated at the boil for a further 3 h, hydrolysed by addition of water and worked up by extraction in a conventional manner. The product is isolated by chromatography on silica gel using pentane, and the pure E material is obtained by preparative HPLC on RP silica gel using an acetonitrile/water mixture.

C 25 I; Δn=0.0960; Δ∈=19.1; γ₁=38

The following compounds of the formula

are prepared analogously:

L¹ L² L³ L⁴ X H H H H F F H H H F F F H H F F F F H F F H F H F F H F F F H H F F F H H H H OCF₃ F H H H OCF₃ F F H H OCF₃ F F F H OCF₃ F F F F OCF₃ F H F H OCF₃ F H F F OCF₃ H H F F OCF₃ H H H H Cl F H H H Cl F F H H Cl F F F H Cl F F F F Cl F H F H Cl F H F F Cl H H F F Cl H H H H CN F H H H CN F F H H CN F F F H CN F F F F CN F H F H CN F H F F CN H H F F CN H H H H OCHF₂ F H H H OCHF₂ F F H H OCHF₂ F F F H OCHF₂ F F F F OCHF₂ F H F H OCHF₂ F H F F OCHF₂ H H F F OCHF₂ H H H H OCH═CF₂ F H H H OCH═CF₂ F F H H OCH═CF₂ F F F H OCH═CF₂ F F F F OCH═CF₂ F H F H OCH═CF₂ F H F F OCH═CF₂ H H F F OCH═CF₂ F H H H CF₃ F F H H CF₃ F F F H CF₃ F F F F CF₃ F H F H CF₃ F H F F CF₃ H H F F CF₃ H H H H CF₃

The following mixture examples are intended to explain the invention without restricting it.

Above and below, percentage data denote percent by weight. All temperatures are indicated in degrees Celsius. m.p. denotes melting point, cl.p.=clearing point. Furthermore, C=crystalline state, N=nematic phase, S=smectic phase and I=isotropic phase. The data between these symbols represent the transition temperatures. Furthermore,

-   -   Δn denotes the optical anisotropy at 589 nm and 20° C.,     -   γ₁ denotes the rotational viscosity (mPa·s) at 20° C.,     -   V₁₀ denotes the voltage (V) for 10% transmission (viewing angle         perpendicular to the plate surface), (threshold voltage),     -   Δ∈ denotes the dielectric anisotropy at 20° C. and 1 kHz         (Δ∈=∈_(∥)−∈_(⊥), where ∈_(∥) denotes the dielectric constant         parallel to the longitudinal axes of the molecules and ∈_(⊥)         denotes the dielectric constant perpendicular thereto).

The electro-optical data are measured in a TN cell at the 1st minimum (i.e. at a d·Δn value of 0.5 μm) at 20° C., unless expressly indicated otherwise. The optical data are measured at 20° C., unless expressly indicated otherwise. All physical properties are determined in accordance with “Merck Liquid Crystals, Physical Properties of Liquid Crystals”, status November 1997, Merck KGaA, Germany, and apply for a temperature of 20° C., unless explicitly indicated otherwise.

Example M1

PCH-5F 10.00% Clearing point [° C.]: 92.5 PCH-6F 8.00% Δn [589 nm, 20° C.]: 0.0968 PCH-7F 6.00% Δε [1 kHz, 20° C.] +5.3 CCP-2OCF₃ 8.00% V₁₀ [V]: 2.01 CCP-3OCF₃ 12.00% CCP-4OCF₃ 7.00% CCP-5OCF₃ 11.00% BCH-3F.F 12.00% BCH-5F.F 10.00% ECCP-3OCF₃ 5.00% ECCP-5OCF₃ 5.00% CBC-33F 2.00% CBC-53F 2.00% CBC-55F 2.00%

Tilt of Singles 10% in Mixture M1 in TN Cells

The above illustration shows that the alkenyl compounds of the formula I, such as, for example, UQU-1V-F, disproportionately increase the pretilt in Mixture M1 compared with the corresponding alkyl compounds.

Example M2

CC-3-V 32.00% Clearing point [° C.]: 80.5 UQU-1V-F 5.00% S → N [° C.]: −40 PGP-2-2V 2.00% Δn [589 nm, 20° C.]: 0.1297 CCP-V-1 10.00% Δε [1 kHz, 20° C.]: +16.1 PGU-3-F 7.00% V₁₀ [V]: 1.05 BCH-3F.F.F 10.00% APUQU-3-F 9.00% PGUQU-3-F 5.00% PGUQU-4-F 6.00% PGUQU-5-F 7.00% DPGU-4-F 7.00%

Example M3

CC-3-V 28.50% Clearing point [° C.]: 82.0 UQU-1V-F 10.00% S → N [° C.]: −30 PGP-2-2V 1.00% Δn [589 nm, 20° C.]: 0.1304 CCP-V-1 12.50% Δε [1 kHz, 20° C.]: +15.4 PGU-3-F 6.00% BCH-3F.F.F 8.00% APUQU-3-F 8.00% PGUQU-3-F 5.00% PGUQU-4-F 5.00% PGUQU-5-F 6.00% DPGU-4-F 7.00% CPGP-4-3 3.00%

Example M4

CC-3-V 30.00% Clearing point [° C.]: 81.0 UQU-1V2-F 5.00% S → N [° C.]: −30 PGP-2-2V 1.00% Δn [589 nm, 20° C.]: 0.1300 CCP-V-1 13.00% Δε [1 kHz, 20° C.]: +15.8 PGU-3-F 9.00% V₁₀ [V]: 1.09 BCH-3F.F.F 8.00% APUQU-3-F 7.00% PGUQU-3-F 5.00% PGUQU-4-F 6.00% PGUQU-5-F 8.00% DPGU-4-F 8.00% 

1. Liquid-crystalline medium, characterised in that it comprises one or more compounds of the formula I,

in which X denotes F, Cl, CN, OCN, SF₅, a fluorinated alkyl radical having 1-6 C atoms, a fluorinated alkoxy radical having 1-6 C atoms, a fluorinated alkenyl radical having 2-6 C atoms, a fluorinated alkylalkoxy radical having 1-6 C atoms, L¹⁻⁴ each, independently of one another, denote H or F, and alkenyl denotes an alkenyl radical having 2-6 C atoms.
 2. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds of the formulae II and/or III:

in which A denotes 1,4-phenylene or trans-1,4-cyclohexylene, a denotes 0 or 1, R³ denotes alkenyl having 2 to 9 C atoms, and R⁴ denotes a straight-chain alkyl radical having 1 to 12 C atoms or alkenyl radical having 2 to 9 C atoms.
 3. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from the compounds of the formulae

in which R^(3a) and R^(4a) each, independently of one another, denote H, CH₃, C₂H₅ or C₃H₇, and “alkyl” denotes a straight-chain alkyl group having 1 to 8 C atoms.
 4. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from the compounds of the formulae IV to VIII:

in which R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH₂ groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF₂O —, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinated alkenyl or alkenyloxy radical having 2 to 6 C atoms. Y¹⁻⁶ each, independently of one another, denote H or F, Z⁰ denotes —C₂H₄—, —(CH₂)₄—, —CH═CH—, —CF═CF—, —C₂F₄—, —CH₂CF₂—, —CF₂CH₂—, —CH₂O—, —OCH₂—, —COO—, —CF₂O— or —OCF₂—, in the formulae V and VI also a single bond, and r denotes 0 or
 1. 5. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from the compounds of the formulae Va to Vj:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH₂ groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinated alkenyl or alkenyloxy radical having 2 to 6 C atoms.
 6. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from the compounds of the formulae VI-1a to VI-1d:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH₂ groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinated alkenyl or alkenyloxy radical having 2 to 6 C atoms.
 7. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from the compounds of the formulae VI-2a to VI-2f:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH₂ groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinated alkenyl or alkenyloxy radical having 2 to 6 C atoms.
 8. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from compounds of the formulae X and/or XI:

in which R⁰ and X⁰ have the meanings indicated below, R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH₂ groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinated alkenyl or alkenyloxy radical having 2 to 6 C atoms. Y¹⁻⁴ each, independently of one another, denote H or F, und

each, independently of one another, denote


9. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from compounds of the formula XII,

in which R¹ and R² each, independently of one another, denote n-alkyl, alkoxy, oxaalkyl, fluoroalkyl, alkenyloxy or alkenyl, each having up to 9 C atoms, and Y¹ denotes H or F.
 10. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more compounds selected from the following formulae:

in which R⁰, X⁰, Y¹ and Y² have the meanings indicated below, R⁰ denotes an alkyl or alkoxy radical having 1 to 15 C atoms, where, in addition, one or more CH₂ groups in these radicals may each be replaced, independently of one another, by —C≡C—, —CF₂O—, —CH═CH—,

—O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, and in which, in addition, one or more H atoms may be replaced by halogen, X⁰ denotes F, Cl, a mono- or polyfluorinated alkyl or alkoxy radical having 1 to 6 C atoms, a mono- or polyfluorinated alkenyl or alkenyloxy radical having 2 to 6 C atoms, Y¹ denotes H or F Y² denotes H or F.
 11. Liquid-crystalline medium according to claim 1, characterised in that it comprises 0.5-10% by weight of compounds of the formula I.
 12. Liquid-crystalline medium according to claim 1, characterised in that it additionally comprises one or more UV stabilisers and/or antioxidants.
 13. A liquid-crystalline medium according to claim 1 within an electro-optical device.
 14. A liquid-crystalline medium according to claim 13 within an active-matrix display, passive-matrix display or a liquid-crystal lens.
 15. Electro-optical liquid-crystal display containing a liquid-crystalline medium according to claim
 1. 16. Process for the preparation of a liquid-crystalline medium according to claim 1, characterised in that one or more compounds of the formula I are mixed with at least one further mesogenic compound and optionally additives.
 17. Compounds of the formula I,

in which X denotes F, Cl, CN, OCN, SF₅, a fluorinated alkyl radical having 1-6 C atoms, a fluorinated alkoxy radical having 1-6 C atoms, a fluorinated alkenyl radical having 2-6 C atoms, a fluorinated alkylalkoxy radical having 1-6 C atoms, L¹⁻⁴ each, independently of one another, denote H or F, and alkenyl denotes an alkenyl radical having 2-6 C atoms.
 18. Compounds of the formulae I1 to I5:

in which X and L¹⁻⁴ have the meanings indicated in claims
 17. 